Plane-iron mini-mystery

My parents are visiting for the Thanksgiving week, and over the weekend my dad presented me with an old Stanley Bailey No.4 hand plane that used to belong to his dad (and maybe his dad’s dad before him);

Even though it’s a Sweetheart plane from the ‘20s or ‘30s, the top end of the plane iron is heavily mushroomed, as if someone had repeatedly used a hammer to adjust the depth of cut. On the other hand, the plane iron appears to be full length; the iron cannot be raised any higher by the brass adjustment wheel, so it appears to have been sharpened very little.

So there’s the mystery—the iron length indicates very little usage (and the whole plane shows very little wear), but the mushroomed end indicates it may have been used a lot.